Russia's prosecutor-general has opened a criminal case against the UK-based Russian tycoon Boris Berezovsky, accusing him of a coup plot.

Russia has sent an extradition request to the UK government.

Mr Berezovsky, 59, was granted asylum three years ago and has a fortune estimated at £800m ($1.4bn).

He told a Russian radio station in January he wanted to replace what he called the "anti-constitutional regime" of President Vladimir Putin.

British warning

On Tuesday, the UK Foreign Secretary, Jack Straw, warned Mr Berezovsky that his refugee status could be reviewed, in light of his call for the overthrow of President Putin.

Mr Berezovsky, an ally of the former President, Boris Yeltsin, has already fought off extradition requests on fraud charges which he said were politically motivated.

Responding to the latest extradition request, he told Ekho Moskvy radio that the only anti-constitutional role he had played was in helping Vladimir Putin to become president.

In an interview with Ekho Moskvy on 25 January, the tycoon said President Putin was "violating the constitution and, today, any forceful actions by the opposition will be justified".

"That includes a forceful seizure of power, and that's what I've been working on," he said.

He said President Putin's "regime has lost all legitimacy" and was "leading Russia into the abyss".

Kremlin influence

The prosecutor-general's office said the Federal Security Service (FSB) - the main successor to the Soviet KGB - was pursuing the case against Mr Berezovsky.

Using London as his base Mr Berezovsky has actively supported pro-democracy groups opposed to President Putin.

Mr Berezovsky, a mathematician turned billionaire businessman, was a key power-broker in the Yeltsin administration and was instrumental in bringing Vladimir Putin to power, the BBC's Steven Eke reports.

However, he was one of the first targets of President Putin's crackdown on the Russian oligarchs, and went into self-imposed exile abroad at the end of 2000.